In these days, it is expected that on-line multi-media contents selling methods will be widely used. In such on-line selling methods, moving-picture data or audio data are delivered to a buyer's recording medium such as a writable CD or DVD through a data network and are then played back from that recording medium.
The audio data to be provided via a network are called EMD (Electronic Music Distribution) audio data. EMD audio data are provided by a lot content providers. However, the content providers encode END audio data to mutually different formats and a different unique copy-protection keys, namely, encryption keys to EMD audio data as well to prevent unauthorized copy and to protect their copyright.
A decryption key adequate for downloaded EMD audio data must be obtained from a contents provider having provided the EMD audio data before playing the downloaded EMD audio data. The decryption key is used for decrypting EMD audio data encrypted with a corresponding copy-protection key.
Because it is nearly impossible to obtain an adequate decryption key without lawful approval, EMD audio data can be protected against unauthorized copy, thus its copyright can be protected well.
To enjoy such a data protecting advantage, high-quality moving-picture data and/or audio data will be encrypted in general to sell them to buyers through a network. In addition, because of such a data protecting advantage, on-line content selling methods are expected to be spread more and more.
However, as aforementioned, encrypted EMD audio data must require a copy-protection key, namely, a decryption key to be played back. Therefore, a user can not play with a disk device any piece of EMD audio data downloaded onto a writable recording medium before he or she purchases a decryption key. In other words, a user can not confirm at all whether the downloaded EMD audio data are what he or she wants or whether its audio quality is better or not than he or she expected. Consequently, a user hesitates to buy a play-authorizing key, namely, a decryption key for the downloaded EMD audio data.
In the meantime, when a user wants to buy his or her favorite digital song or music, he or she makes web-surfing on Internet to find a web site and then searches the web site for his or her favorite song or music. If found, he or she requests it to be downloaded to a recording medium. Theses procedures take much time and effort, which would impede spread of the on-line selling methods of digital contents.